1. Field of the Invention
This invention relates to any system using a robust OFDM transmission scheme which is capable of reliably carrying a plurality of services or programs (referred to collectively herein as services) within each channel of allocated bandwidth even in a selective channel fading environment.
2. Background of Related Art
A transmission channel used by an In-Band On-Channel (IBOC) FM Digital Audio Broadcast (IBOC FM-DAB) system allows simultaneous transmission of Digital Audio Broadcast (DAB) information and Frequency Modulation (FM) over existing FM frequency channels without interfering with conventional analog FM signals.
For instance, within each transmission channel (e.g., each FM station) having an allotted station bandwidth of, e.g., 200 kHz, a number of different DAB services or programs (e.g., three different DAB services or program information bit streams from three different sources) can be transmitted simultaneously using Orthogonal Frequency Division Multiplexing (OFDM) modulation. OFDM has several desirable properties, e.g., it simplifies or even eliminates equalization problems considerably, has graceful performance degradation, and because of the absence of equalization, can be lower in complexity.
In OFDM, different bits are placed in each of the available OFDM subcarriers 1 to N, e.g., bits 1 to 4 of a data sample in one OFDM subcarrier, bits 5 to 8 of the data sample in another OFDM subcarrier, etc., for all of M bits in N OFDM subcarriers. Note that this is similar to a frame, but in the frequency domain, not in the time domain.
FIG. 4 shows a block diagram of a conventional OFDM transmission system.
In particular, in the transmitting portion 420 of the OFDM transmission system shown in FIG. 4, a data source containing source bits from all the different DAB services or programs (e.g., three different DAB services or programs) is input to a bits to symbol mapper 402. The bits to symbol mapper 402 maps the data bits for current frames output from respective DAB services or programs into a contiguous symbol stream. The contiguous symbol stream is input to an OFDM transmitter and radio frequency (RF) transmitter front end 404, which transmits the contiguous symbol stream at various available OFDM subcarrier frequencies.
At the receiving portion 430 of the OFDM transmission system shown in FIG. 4, an RF receiver front end and OFDM receiver 406 receives the contiguous symbol stream containing the information for the number of different DAB services or programs (e.g., for three different DAB services or programs). A symbols to bits demapper 408 converts the contiguous symbol stream back into a data bit stream containing data for the current frames of all the different DAB services or programs.
FIG. 5 shows a conventional distribution of OFDM subcarriers 1 to N for use by the different DAB services or programs, e.g., three different DAB services or programs 502-506. The OFDM subcarriers 1 to N are used in the transmission between the transmitting portion 420 and receiving portion 430 of a conventional OFDM transmission system.
When a number of different DAB services or programs 502-506, e.g., three, are simultaneously transmitted, the available OFDM subcarriers 1 to N are conventionally distributed or assigned contiguously between the three different DAB services or programs 502-506. The number of subcarriers assigned to each DAB service or program is application specific, and typically depends upon the information capacity required by each DAB service or program.
In conventional OFDM transmission systems, each of the different DAB services or programs are typically assigned a fixed, contiguous subset of the total number of available OFDM subcarriers 1 to N. Thus, a first DAB service or program is typically assigned the first N1 of all available OFDM subcarriers (e.g., 1 to N1), a second DAB service is typically assigned the next N2 of the remaining available OFDM subcarriers (e.g., N1+1 to N2), etc.
For instance, as shown in FIG. 5, a first DAB service 502 includes contiguous OFDM subcarriers 1 to N1, a second DAB service 504 includes contiguous OFDM subcarriers N1+1 to N1+N2, and a third DAB service 506 includes contiguous OFDM subcarriers N1+N2+1 to N. Thus, of an available group of OFDM subcarriers 1 to N and three DAB services or programs, OFDM subcarriers 1 to N1 are conventionally assigned for use by the first DAB service 502, OFDM subcarriers N1+1 to N1+N2 are conventionally assigned for use by the second DAB service 504, and contiguous OFDM subcarriers N1+N2+1 to N1+N2+N3 (i.e., N) are conventionally assigned for use by the third DAB service 506.
However, a transmission channel 540 (e.g., an FM station) containing the OFDM subcarriers 1 to N of the different DAB services or programs 502-506 may be subject to slow selective fading affecting some of the DAB services or programs but not others, particularly in a fixed or slow speed mobile environment. For instance, one DAB service or program may be detrimentally affected at the same time that a second DAB service or program may be unaffected.
Each of the different DAB services or programs 502-506 conventionally may have its own interleaver and/or Forward Error Correction (FEC) scheme to improve the quality of the transmission channel 540. Thus, particularly with the implementation of such error correction schemes, it is recognized by the present inventors that unaffected DAB services or programs in light of selective channel fading may have excess margin to more than compensate for the channel fading for that respective DAB service or provider. In this case, while the interleaver and/or FEC function of the second DAB service or program may be suitable to maintain reliable communications in the transmission channel 540, e.g., an FM station, the interleaver and FEC function of the first DAB service or program may not be adequate to fully overcome the deterioration of the data communication due, e.g., to the channel fades. Thus, the impact of channel fades may not be fully or evenly mitigated by all or substantially all DAB services or programs within any one transmission channel.
One of the DAB services or programs may be hit by the fade much more severely than another. For instance, a portion or all of one DAB service or program may be affected whereas at the same time another DAB service or program may be marginally affected or not affected at all. In such an environment, the OFDM transmission scheme in the transmission channel 540 may be adequate for one DAB service or program yet at the same time be inadequate for another.
There is thus a need to improve the reliability of all or substantially all DAB services or programs transmitted within a transmission channel, e.g., within an FM channel, using OFDM modulation.
In accordance with the principles of the present invention, a transmitter for an OFDM transmission system comprises a data reassignment encoder and an OFDM transmitter. The data reassignment encoder shuffles an input data stream corresponding to a plurality of services into a shuffled output data stream, the shuffled data in the output data stream being shuffled to effectively reassign each of the services with respect to an arrangement of corresponding data in the input data stream. The OFDM transmitter transmits the output data stream at a plurality of subcarriers.
A receiver is also provided. The OFDM receiver is adapted to receive a shuffled data stream corresponding to a plurality of services. A data reassignment decoder un-shuffles the shuffled data stream into an unshuffled data stream to effectively re-align the data stream with respect to the services.
A method of transmitting a data stream corresponding to a plurality of services using OFDM is also provided in accordance with the present invention. The method comprises shuffling an input data stream corresponding to the DAB services into an output data stream containing shuffled data with respect to corresponding data in the input data stream. The shuffled data stream is transmitted over a plurality of subcarriers using OFDM. The shuffling of the input data stream effectively reassigns each of the subcarriers with respect to the services.
A method of receiving a shuffled data stream corresponding to a plurality of services using OFDM is also provided. This method comprises receiving a shuffled input data stream transmitted over a plurality of subcarriers using OFDM. The input data stream is un-shuffled, and an output data stream is provided with respect to a plurality of services. The un-shuffling is based on a control signal received with the shuffled input data stream, and effectively re-aligns a data stream with respect to the plurality of services.